1. Field of the Invention
The present invention relates to a device for reducing the relative humidity of a flowing gas which is warmer than an ambient atmosphere, such as gas expired by a live subject of the type having an inlet for the flowing gas, at least one flow-through channel in which the flowing gas is dehumidified and an outlet for the flowing gas.
2. Description of the Prior Art
Dehumidification of a humid flowing gas is important in many contexts. For example, in conjunction with measurement of the flow of flowing gases, many flow meters operate less effectively, and may even produce erroneous measurements, when the flowing gas is so humid that liquid water accumulates in the flow meter by condensation of water vapor. Other measurement instruments, such as various gas analyzers, are also sensitive to condensation inside the measurement instrument. Preventing condensation of water vapor into liquid, which would accordingly affect a measurement instrument, is especially important in the respirator treatment of patients, since the respirator's function is based on correct operation of various measurement instruments. Gas expired by the patient is generally saturated with water vapor and also has a relatively high temperature, about 36.degree. C.
Ambient temperature around the patient where the measurement equipment is located is usually in the range of 18.degree.-22.degree. C. Cooling of expired air is generally unavoidable, and condensation therefore forms.
One known device for dehumidifying air expired by a patient is described in the brochure "Star Exhalation Isolation System Operating Instructions, Siemens Version, Infrasonic Inc., Form No. 9910053, April 1988. The device is connected to the expiratory outlet of a ventilator in order to warm expired gas when it passes through the device. When expired air is warmed, the relative humidity of the expired air decreases thereby reducing the risk of condensation inside a measurement instrument. The reduction of relative humidity occurs because warm air has a higher saturation vapor pressure. One disadvantage of this known device is that it requires an external power source and some type of element for heating the expired air. Moreover, the device must be placed close to the measurement instrument, since the heated gas would otherwise have time to cool again before reaching the measurement instrument.
Another version of a device which reduces the risk of condensation of water vapor in expired air is described in an article entitled "Expired Gas Cooling Device" by J. Attwood and L. Bartel. This device includes a cooling unit for condensing water vapor in expired air before it reaches the measurement instrument. The expired air is cooled in order to reduce the risk of condensation inside the measurement instrument, even if air entering the measurement instrument may still have a very high relative humidity and may even still be saturated. Since a gas with a low temperature cannot hold as much water vapor as a warmer gas, relative humidity as such is not actually reduced when expired air is cooled. There is, however, no additional cooling of the expired air and the risk of condensation inside the measurement instrument is therefore reduced.
The cooling device also requires some external source of power. In this instance, a fan is powered which blows air across a radiator system through which expired gas passes.
In addition to the fact that the described devices stop working when their power source (electric power supply) fails, the devices are also clumsy to use and difficult to clean.